CAM plants: photosynthesizing in the desert

Although C4 plants are well adapted to occasional periods of drought, they cannot cope well with desert conditions. A group of plants including cacti and pineapples have evolved a third type of carbon dioxide fixation which enables them to survive in very dry climates. These plants are called CAM plants. CAM is an abbreviation for crassulacean acid metabolism, a type of metabolism first observed in the family of plants called Crassulaceae (which includes the stonecrops, fleshy-leaved plants that will grow on rocks and walls).

CAM plants conserve water by only opening their stomata at night. During the night, they fix carbon dioxide into oxaloacetate which is converted into malate. This acts as a carbon dioxide storage compound. During the day, malate releases carbon dioxide into the Calvin cycle. This allows photosynthesis to take place on hot, dry, sunny days, even though the stomata are closed.

CAM plants conserve water very well and are able to survive in extremely dry conditions, but CAM plants do not photosynthesise very efficiently. Most are very slow growing. Where there is plenty of water, CAM plants cannot compete well with C3 and C4 plants.

CAM plants and C4 plants have a similar metabolism: carbon dioxide is first fixed into a four-carbon intermediate before it enters the Calvin cycle. However, in CAM plants the initial fixation and the Calvin cycle occur at separate times, whereas in C4 plants the initial fixation and the Calvin cycle are separated structurally but both occur during the day. C4 plants live in hot, very sunny, and periodically dry environments but where lack of water is rarely a limiting factor (partly because the plants can reduce water losses due to their C4 metabolism) and annual rainfall is high (typically, tropical rainforest-type climates); CAM plants are desert plants that live in areas of very low annual rainfall. Note that C3, C4, and CAM plants all eventually use the Calvin cycle to make glucose from carbon dioxide.

■ Glossary of essential terms for you to know

English term Russian equivalent
to thrive разрастаться
to depend on зависит от
to adapt приспособиться
to survive выживать
to release выпускать
to breed размножаться
to cope with справиться с
photorespiration световое дыхание
to fix закрепить, фиксировать
compound смесь
to relate to относиться к
efficiently продуктивно, эффективно
temperate умеренный
to suffer страдать
sufficient достаточный, подходящий
to escape убежать
to cease прекращать
leaf листок
to wilt ослабевать
to decrease уменьшаться
to combine объединять
loss потеря
to reduce уменьшать
to evolve развивать
to carry out выполнять
bundle sheath обкладка сосудистых пучков
vein вена, жилка
to surround окружать
cell клетка
initial первоначальный
pathway тропа, путь
carboxylase карбоксилаза
to increase увеличивать
to enable позволять
to compete конкурировать, соперничать
maize кукуруза
sugar cane сахарный тростник
millet просо
drought засуха
desert пустыня
rock горная порода, скала
to conserve сохранять
to enter входить
to occur происходить
lack of недостаток
annual ежегодный
rainfall осадки
rainforest тропический лес

■ Your Essential Assignments

I. Quick check

1. Name two C3 plants.

2. Why is sugar cane called a C4 plant?

3. When do CAM plants fix carbon dioxide?

4. Suggest which type of carbon dioxide fixation (C3, C4, or CAM) is most efficient:

a. If it is not hot and sunny and the carbon dioxide level is low, but water is freely available

b. In hot, dry and sunny climates where stomata are closed

c. In bright light and temperate regions where there is an ample water supply.

II. Fill in the missing words:

Term (verb) Noun Adjective
adapt ....... .......
fix ....... .......
conserve ....... .......
separate ....... .......
dry ....... .......

III. Use monolingual English dictionary and write down what could the words given below mean:

adapt, hot, dry, condition, growth, compound.

IV. Match these words with their definitions:

adaptation A. happening at the beginning, first
metabolism B. a large amount of something in one place or area
cycle C. the gas produced when animals breathe out, when carbon is burned in air or when anima or vegetable substances decay
initial D. a plant such as wheat, rice or fruit that is grown by farmers, especially in order to be eaten
sufficient E. the state of not having something, or not having enough of it
wilt F. chemical activity in your body that uses food to produce the energy you need to work and grow
7. concentration G. To continue to exist in spite of many difficulties and dangers
8. desert H. To succeed in dealing with a difficult problem or situation
9. carbon dioxide I. the smallest part of a living thing that can exist independently
survive J. As much as is needed for a particular purpose; enough
crop plant K. To stop doing something or happening
12. cope with L. a number of events happening in a regularly repeated order
13. cell M. if a plant wilt, it bends over because it is too dry or old
14. cease N. the process by which something changes or is changed so that it can be used in a different way or in different conditions
15. lack of O. a large area of sand where it is always very hot and dry

V. Find English equivalents to the following word combinations:

Russian term English equivalent
1. жаркий и сухой экваториальный регион  
2. замерзающий полярный регион  
3. пшеница, соевые бобы и рис  
4. умеренные условия  
5. маленькие поры в листьях  
6. уменьшает световое дыхание  
7. приспособлены, чтобы выполнять  
8. уменьшает потери воды  
9. высокая концентрация углекислого газа  
10. справляться с условиями пустыни  
11. выжить в очень сухом климате  

VI. Give Russian equivalents to the following English terms:

English term Russian equivalent
thrive in environments  
cope with the demands  
ways of fixing carbon dioxide  
common and widely distributed  
hot, dry environments  
obtain sufficient carbon dioxide  
allow water to escape  
cease photosynthesising  
plant growth  
the loss of  
a special metabolic adaptation  
leaf veins  
the metabolic pathway  
the bundle sheath  
photosynthetic efficiency  
survive in extremely dry conditions  
occur at separate times  

VII. Find synonyms among the pool of words:

Pool of words Synonyms
1)1.thrive/2.lack/3.grow/4.absence/5.cycle/6.rotation  
2)1.damage/2.harm/3.cease/4.condition/5.situation/6.finish  
3)1.efficient /2.sufficient /3.ample /4.productive  
4)1.annual /2.drought /3.lack of rain /4.once a year  

VIII. Answer the following questions. Use all information given before:

1. Where do green plants thrive?

2. How do C3 plants fix carbon dioxide?

3. What are two major disadvantages of C3 plants in hot, dry environments?

4. How do C4 plants fix carbon dioxide?

5. What are the bundle sheath cells?

6. What is the Hatch-Slack pathway?

7. What are two main advantages of C4 plants in hot, dry environments?

8. What are CAM plants?

9. Why can CAM plants survive in very dry climates?

IX.Match the sentence halves. Make complete sentences:

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